1. Field of the Invention
This invention relates to a process for producing a resin-metal laminate material, and more specifically to an extrusion laminate process for producing a resin-metal laminate material, especially a metal laminate material coated with a resin on both surfaces, in which a resin coating is a thin film having high performances such as the uniformity of thickness, high processability, high adhesion properties and high film properties, at a high speed.
2. Description of Prior Art
It has heretofore been widely practiced to coat a metal surface with a resin layer as means for imparting corrosion resistance to a metal material. The coating methods used in this technique include a method of painting a dispersion of a thermosetting resin such as an epoxy resin, a phenol resin, an acrylic resin or a polyester resin in a solvent on a metal surface and a method of adhering a pre-formed film of, for example, a polyester type, an olefin resin type, or a polyamide type to a metal substrate through an adhesive such as an isocyanate type, an epoxy type or a phenol type.
It is well known to utilize the heat fusion of a thermoplastic resin in adhering a metal substrate to the thermoplastic resin. This technique includes a method of adhering a pre-formed film of, for example, a thermoplastic polyester to a metal plate by heat adhesion, and a method of adhering a molten thin film of an extruded thermoplastic polyester resin to a metal plate.
Japanese Laid-Open Patent Publication No. 137760/1976 describes a coating apparatus for coating a synthetic resin on both surfaces of a sheet such as paper or an aluminum foil comprising two T dies standing opposite to each other and a sheet feeding apparatus provided to travel the sheet between the two T dies, characterized in that during the travel of the sheet, the synthetic resin is extruded from the two T dies to form synthetic resin layers simultaneously on both sides of the sheet. FIGS. 1 and 4 of this patent publication show that after the synthetic resin layers are extruded onto the sheet, the resin layers are passed between the quenching rolls.
U.S. Pat. No. 5,407,702 describes a method of coating by extruding a resin on both surfaces of a metal strip while making a film. It is described that the metal strip of an aluminum alloy is moved through a preliminary conditioner, two extrusion dies, an after-heating machine and a quenching system, and both surfaces of the metal strip are coated with a thin coating of a polyester material. In the apparatus shown in FIG. 1 of this patent, it is described that a thin film of the polyester extruded from the die is elongated into a thin film by a first roll, and cooled by a second roll, and is adhered to the heated metal strip by a third roll.
Japanese Laid-Open Patent Publication No. 79801/1994 describes a process of producing a metal plate coated with a resin on both surfaces, which comprises pressing a press-contacting roll on a pre-heated metal plate which is wound on a winding roll, flowing down a molten thermoplastic resin from a T die via an extruder into a gap between the press-adhering roll and the metal plate to provisionally adhere and coat the thermoplastic resin on the metal plate, winding this resin-coated metal plate on another wind-up roll so that the resin-coated surface contacts the wind-up roll side, pressing another press-contacting roll from the side of the metal plate, flowing down a molten thermoplastic resin from a T die via an extruder into a gap between the other press-contacting roll and the metal plate to provisionally adhere and coat the thermoplastic resin to the other surface of the metal plate to give a coated metal plate in which both surfaces are coated with the resin and thereafter, heating the coated metal plate in which both surfaces are coated with the resin by a heating apparatus located downstream.
However, these known techniques are still sufficiently satisfactory to produce a resin metal laminate, in which a resin coating is a thin film and has high performances such as the uniformity of thickness, high processability, high adhesion property and high film properties, at a high speed.
The above cited first technique may be applied to the production of a laminate of a soft wrapping material typified by a pouch, but cannot be applied to the production of a can-making laminate material. In a soft wrapping material laminate, the metal is a very thin metal foil which is intended to impart gas barrier property, but other resin layer is a thick layer which can impart heat sealability and also act as a stress carrier. On the other hand, in a can-making laminate material, the metal can act as a stress carrier, and is subjected to various processings such as press processing, deep draw formation, bend-elongation processing, and ironing working. Furthermore, the resin coated layer must be thin and within a range in which corrosion resistance, adhesiveness and the uniformity of the film are ensured with respect to processability. It is difficult to use the above-mentioned first technique to apply a thin resin film onto a metal surface, and thus, is not suitable for the production of a can-making laminate material.
The cited second and third techniques can be recognized to be applicable to the production of a can-making laminate material. From the viewpoint of producing a resin metal laminate material having high performances, these techniques are still not satisfactory. These techniques require the operation of heating a metal plate prior to the lamination of a resin such as a polyester, and the operation of heating the resin metal laminate material after the lamination of the resin to complete the fusion and adhesion of the resin. The operation of heating the metal plate or the resin-coated metal plate several times at a high temperature of more than the melting point of the polyester causes the heat softening of the metal plate and the deterioration of the resin by heat decomposition or heat oxidation, and results unpreferably in the lowering of the various properties of the laminate material. The lowering of the properties of the laminate material becomes marked as the time of heating is frequent, and as the thickness of the thin resin film is generally decreased.
Furthermore, in the production of a can-making resin metal laminate material, there is a technical problem in which a thin resin film should be firmly adhered to a metal plate with a uniform thickness. For example, in the case of a film biaxially stretched in advance, a laminate by heat-adhering in a relatively uniform thickness may be possible. But a film should be formed and stretched in a separate step, and the steps may become complicated. On the other hand, in the case of an extruded coating in the above-cited second technique, complicated operations of cooling and film formation must be carried out while an extruded molten resin is elongated in a thin film. At the time of film formation, the surface temperature of the resin is lowered, and a firm heat film adhesion to a metal plate becomes difficult. Furthermore, as the film becomes thinner and travels at a higher speed, the occurrence of creases of the resin film on the third roll may be apprehended.